1. Field of Invention
The invention relates to photolithography, and more particularly to gray scale photolithography used to define edges on microelectronic device patterns during integrated circuit fabrication.
2. Related Art
Photolithography is used in semiconductor integrated circuit fabrication. Direct writing photolithography is typically used to define reticle mask patterns subsequently employed during projected image photolithography printing on the integrated circuit wafer. In direct writing (two-step) lithography strategies, radiated energy, such as laser or electron beams, is directed in a controlled manner on to an energy sensitive layer (e.g., a photoresistive layer or xe2x80x9cresistxe2x80x9d) overlying a substrate. The energy beams expose a pattern in the energy sensitive layer which, depending on the composition of the energy sensitive layer, becomes more soluble (positive resist) or less soluble (negative resist) in a chemical developer solution. The amount of accumulated energy from one or more beams incident on a location determines the exposure amount.
The pattern to be printed is typically defined in an orthogonal array of picture elements (pixels) oriented in X (rows) and Y (columns). Thus each pixel in the array has a unique address location. A pattern is defined by exposing selected pixels in a single array, or by exposing selected pixels in two or more overlapping and/or interstitial arrays. The distance between two adjacent pixels in an array is the pixel pitch.
In some conventional printing (imaging or writing) strategies, each pixel receives either the maximum beam energy or no beam energy. In enhanced printing strategies each pixel may receive one of several intermediate (between zero and maximum) xe2x80x9cgray levelxe2x80x9d beam energy levels (intensities or doses). In a typical gray level strategy, each gray level number represents a corresponding beam intensity that is linearly proportional to the gray level number. For example, in a gray level printing strategy using seventeen gray levels (0-16), gray level number 1 represents one-sixteenth of the maximum beam energy, gray level number 2 represents two-sixteenths of the maximum beam energy, etc. Thus the intensities corresponding to the gray level numbers linearly increase in proportion to the gray level numbers.
A typical beam energy cross section is approximately gaussian. Maximum energy per unit area occurs at the pixel center and decays with distance from the center. It is therefore helpful to consider a pixel as a particular target location rather than a defined area. To ensure proper energy sensitive layer exposure, pixels are positioned so that the energy each pixel receives from the energy beam overlaps other pixels. Thus the total accumulated energy a particular pixel receives is often the sum of beam energy directed at the particular pixel and beam energies directed at nearby pixels. When a lithographic tool exposes the energy sensitive layer by irradiating a pixel array, the edge of a printed geometric pattern is defined by the total energy received at each pixel.
FIG. 1 illustrates total energy received from an incident beam along a column of pixels 10. The number adjacent each of the six pixels shown represents the gray level number in a strategy using, for example, 17 gray levels. Immediately above column 10 is a graph plotting energy incident at each pixel against position. As discussed above, individual energy curves 12 represent the gaussian energy cross section each of the column 10 pixels receives from the incident beam. Note that the curve representing gray level number 8 is one-half the maximum gray level number 16. No energy is incident on pixels with gray level number 0 (zero). Curve 14 is the sum of curves 12 and represents the total accumulated energy cross section received by the energy sensitive layer. Curve 14 is normalized to have 1.0 as double the intensity required to fully clear the energy sensitive layer.
Persons skilled in the art will understand that assigning various gray level numbers to pixels, and thus changing the associated energy dose directed to corresponding pixels, moves the edge position. In this way small incremental changes are made to edge positions when printing small geometric shapes. By one definition, an edge of an object to be printed occurs at 0.5 on the normalized scale described above. In this model, the slope is steepest at the 0.5 dose, thereby giving the smallest change in critical dimension (CD) or size of the feature per percent dose change. In actual practice, the maximum exposure energy used is often greater or less than the optimal dose to clear because the dose is used to make features larger or smaller. This action sacrifices CD uniformity, however, because systematic and random dose errors will generate larger CD errors than the optimal dose to clear. In FIG. 1, edge position 18 is shown corresponding to the position of the pixel assigned gray level number 8.
The effective grid is the grid on which pattern edges may be defined. Using multiple exposures (printing passes) and/or gray levels per pixel per exposure, the smallest effective grid of the pattern being printed that can be resolved is the pixel pitch divided by the product of the number of exposures and the number of non-zero intensity levels per exposure (assuming typically linear values for intensity levels are fixed for all exposures). On a multiple exposure printing system, such as an Etec Systems, Inc. ALTA 3500, successive exposures may have the center of the pixels shifted in X and Y. In such multiple exposure systems, the effective pixel pitch after one or more exposures would be the distance between the pixel centers. When the dominant distribution of energy per pixel spot size at half power is from 2 to 4 times wider than the effective pixel pitch, pattern edges can be defined at the desired effective grid positions.
FIG. 2 illustrates one gray level scheme similar to that used in an Etec Systems, Inc. CORE 2564 lithography tool in a single exposure, 16 non-zero gray levels per pixel mode to define the edge of an object to be printed. Alternatively, FIG. 2 illustrates, for example, one of 8 exposures made on an ALTA 3000. Numbers shown adjacent each pixel are illustrative and represent the gray level number assigned to the pixel. During printing, an energy dose corresponding to the gray level number is then directed at the pixel. In column 2A edge E0 is centered on the row R1 pixel that is assigned gray level number 8, and consequently receives 0.5 (eight-sixteenths) of the normalized maximum dose. By increasing the row R1 gray level number by one, an action that correspondingly increases beam intensity, the edge is defined slightly higher at E1, as shown in column 2B. The edge is moved in successive small increments by incrementing the row R1 pixel gray level number until the maximum (level 16) is reached. At that point, the row R2 pixel gray level numbers are incremented. Finally, as shown in column 2Q, edge E16 is centered on the row R2 pixel that is assigned gray level number 8, and is displaced by one row from the row 2A position. Thus an edge is positioned between rows R1 and R2 in approximately 16 equal steps.
FIG. 3 illustrates another gray scale printing scheme used in the Etec Systems, Inc. ALTA family of lithography tools. The ALTA 3500 prints patterns using 16 non-zero gray levels per exposure, typically making 4 to 8 exposures to define a complete pattern. Each of the beam intensities associated with the 16 non-zero gray levels may be independently set in the ALTAs. The intensities are typically set in linear proportion to the gray levels. Between exposures the center of each pixel is shifted in X and Y. FIG. 3 illustrates edge placement in one dimension and without coordinate shifting for clarity.
As shown, the scheme uses 16 gray level numbers but moves the edge by alternately incrementing row R1 and row R2 gray level numbers. This is implemented on the ALTA by printing row R1 in one or more exposures and printing row R2 in one or more separate exposures. Thus each edge is defined by pixels in two adjacent rows having received an intermediate energy dose. As in column 2A (FIG. 2), in column 3A edge E0 is centered on the row R1 pixel. In column 3B the row R2 pixel is incremented by one gray level number to define edge El slightly higher (one-sixteenth pixel pitch) than E0. To move the edge another increment, the row R1 pixel gray level number is increased by one to define edge E2 as shown in column 3C. The scheme alternately increments the row R1 and R2 pixels until edge E16 is displaced from the edge E0 position by the distance of a row (pixel pitch), as shown in column 3Q.
Rasterization is the process of converting pattern geometry into an array of pixels each at least one associated gray level number. FIG. 4 is a block diagram showing data path components 28 of a conventional lithography tool. Geometric data 30 contains information describing the pattern to be defined on work piece 32 (e.g., quartz glass covered with a layers of chromium and resist). Data path processor 34 sorts, merges, and transfers to rasterizer 36 the geometries so that different geometry lists can be independently rasterized in parallel for optimal printing. In the Etec Systems, Inc. CORE 2000/2564 systems, for example, the data path processor reformats geometries to a hardware rasterizer geometry instruction format. In the ALTA 3000/3500 systems, the data path processor reformats the geometries to a combined software and hardware instruction format. Rasterizer 36 includes geometry engines 38 and beam circuit boards 40. Geometry engines 38 receive rasterization parameters 42 and convert the geometric information from processor 34 to one or more pixel arrays with gray level numbers associated with each pixel. The pixel array information is passed to beam circuit boards 40 which, in turn, convert the gray level numbers to analog signals that drive accousto-optic modulator 44. Accordingly, gray level numbers associated with each pixel represent, based on parameters from calibrator 54, signals used to modulate one or more energy beams. Laser beam generator 45 directs laser beam 46 into beam splitter 47 that, in turn, creates a plurality (e.g., 32) of separate beams 48. Modulator 44 controls the intensity of each of energy beams 48. Calibrator 54 is coupled to beam boards 40 and is used to set the beam energy level intensity matching each gray level used in the particular printing scheme. Beams 48 are focused and directed by control mechanism 50 to be incident on resist layer 52 overlying work piece 32. Persons familiar with lithography tools will understand the details of both the data path as shown and of similar data paths.
Increasing microelectronic device miniaturization requires that ever-smaller features be printed, and with greater accuracy. Edges must be located as accurately and with as steep a slope as possible. But critical dimensions in present lithography schemes become increasingly nonlinear when submicron (below approximately 500 nanometers (nm)) features are printed. Enhancing edge resolution by increasing the energy dose applied to edge pixels is known, see e.g., U.S. Pat. No. 4,264,711 issued to Greeneich, but requires the data path to first determine edge location before determining an appropriate edge pixel dose. This requirement places an extra computational load on the data path, and requires existing data paths to be redesigned to perform the necessary computational tasks. What is desired is a way using existing system data path architecture to enhance edge resolution while maintaining critical dimension linearity when printing small features.
In accordance with the invention, photolithography embodiments use a gray level printing process in which a conventional set of linearly increasing gray level numbers is defined (e.g., 0-16). A second set of energy beam intensities is defined so that a beam intensity corresponds to each unique gray level number. Unlike other printing strategies, however, the beam intensities are non-linear and non-monotonic in relation to the corresponding gray level numbers (e.g., the beam intensity increase between gray level numbers 2 and 3 may be different from the intensity increase between gray level numbers 3 and 4, and the intensity change between gray level numbers 15 and 16 may be a decrease). In some embodiments one or more beam intensities associated with intermediate gray level numbers are higher than the beam intensity associated with twice the dose to clear a large area on an energy sensitive layer (1.0 on the normalized intensity scale), thereby defining an having a steeper slope.
A geometric shape to be printed (imaged) on the energy sensitive layer is rasterized to produce an array having rows of pixels, and each pixel is associated with at least one gray level number. Printing may be accomplished using multiple printing passes. In some embodiments each printing pass exposes a unique pixel. In other embodiments two or more printing passes expose the same pixel. In some embodiments an edge of the shape is defined by associating intermediate gray level numbers with pixels in two or more rows.